The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to a fuel assembly wherein selected ones of its fuel rods can be removed remotely without disassembling the assembly.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. In the most common configuration, the axis of the core support barrel extend vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles in thereby forming an integral fuel assembly. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween.
It's common, in the startup of new or prototype reactors, to routinely inspect and examine selected ones of the fuel assemblies after stated time intervals of operation. Besides checking for failed fuel rods and examining the wear to the assembly and the deterioration characteristics of the fuel rods themselves, some of the fuel rods may be exchanged with another type of rod wherein a more desired enrichment of the fisson materials is sought. Normally, during operation of a reactor, the fuel assemblies in the active core region are submerged under approximately thirty feet of water, or other suitable coolant, and each fuel assembly is snugly fitted among adjacent assemblies of the organized array. And thus, in order to perform the examination and inspection process, a selected fuel assembly is generally lifted out from its operating position and removed completely out of the reactor or to a convenient work station where the necessary inspection and examination steps are performed while the assembly is still submerged under approximately six feet of collant water. If it is detected that some of the rods have failed or if it is desired to exchange or substitute rods of different composition, it was the past practice to discard the whole fuel assembly and install a new one in its place.
More recently, reconstitutable fuel assemblies have been developed, and rather than discarding the assembly, the fuel assembly can now be refurbished or reconstructed thereby saving many of its expensive components. Conventional reconstitutable fuel assemblies incorporate design features wherein the top nozzle is removed from the assembly in providing access to the fuel rods contained therein. After rod removal and replacement, a new or refurbished top nozzle is secured to the assembly. Due to the space restrictions among the assemblies and the difficulty on working on an assembly located thirty feet under water, reconstitution of a fuel assembly is generally carried out at a remote work station rather than in place. Although reconstitution is less costly than discarding a fuel assembly, it involves labor and down time of the reactor which all equate into added costs.
Thus, in view of the high costs associated with replacing and/or reconstituting fuel assemblies, both domestic and foreign utilities have expressed an interest in fuel assembly, which can be used in the startup of new or prototype reactors, wherein a limited number of fuel rods of preselected fuel assemblies can be removed and replaced remotely for on-site inspection and examination without disassembling the fuel assembly or removing it to another location.